When used as a substrate for thermal-transfer sheet, plastic films, which are weaker to heat, often cause problems of deterioration in releasing and sliding property and breakage of the substrate film because of deposition (sticking) of the film, crust, on the thermal head during printing. For that reason, a method of forming a heat-resistant layer, for example, of a thermosetting resin higher in heat resistance was proposed, but, although the heat resistance is improved, the sliding property of the thermal head is not improved, and a two-liquid-type coating solution should be prepared, because a hardening agent such as crosslinker should be used. In addition, long-term heat treatment (aging) over a period of dozens of hours at relatively low temperature is needed after coating, for preparing a sufficiently hardened film, because the substrate is a plastic thin film that prohibits high temperature treatment. Such a heat treatment makes the production process more complicated and causes problems such as wrinkling during heat treatment and blocking due to adhesion of the coated face to another face in contact, without strict temperature control.
Addition of a silicone oil, low-melting point wax, surfactant, or the like was proposed for improvement in sliding property, but use of an unsuitable lubricant causes problems such as transfer of the thermal-transfer sheet onto the opposite face when the sheet is wound, deposition of buildup on the thermal head during printing, and thus, deterioration in density and definition of the printed image. Although a method of adding a filler for removing the deposit is known, use of an unsuitable filler causes problems such as wrinkling during printing because of increase in the friction coefficient of thermal head and abrasion wear of the thermal head.
To solve these problems above, Patent Documents 1 and 2 disclose a back layer of a silicone-modified polyurethane resin; Patent Document 3, a heat-resistant protective layer of a polysiloxane-polyamine-based block copolymer; and Patent Document 4, a heat-resistant protective layer containing a silicone-modified polyimide resin, but each of the layers had a problem of sticking during high-energy printing because the resin is less heat resistant or a problem in the safety in working environment, demanding an additional exhaust system because of use of a special solvent. Alternatively, Patent Documents 5 and 6 propose a heat-resistant protective layer of a polyamide-imide resin composition, and Patent Document 7 proposes a heat-resistant protective layer containing a polyamide-imide resin and a lubricant, but these layers were insufficient in heat resistance and caused a problem of adverse influence on the printed image by deposition of buildup on the head during high-energy printing.
As shown in FIG. 1, the thermal head commonly used in thermal transfer printer is a thin film-typed head having a heat-releasing substrate 1, and a heat-resistant layer 5, a heat-generating resistor 2, an electrode 3, and an abrasion-resistant layer 4 formed thereon. The heat-releasing substrate 1 is, for example, made of a ceramic material, and the heat-resistant layer 5, which is, for example, made of glass, is formed, as raised on the heat-releasing substrate 1. The thickness of the top area thereof is 20 to 150 μm, and the heat conductivity thereof is approximately 0.1 to 2 Watt/m·deg. The heat-generating resistor 2, which is, for example, made of Ta2N, W, Cr, Ni—Cr, or SnO2, is formed linearly by a thin film-forming method such as vacuum deposition, CVD, or sputtering, and the thickness thereof is approximately 0.05 to 3 μm. The electrode 3, which is, for example, made of Al, is formed on the area of the heat-resistant layer 5 other than the top area, for electrical supply to the heat-generating resistor 2, and the thickness thereof is approximately 0.1 to 34 μm. The abrasion-resistant layer 4 is, for example, made of Ta2O3, SiN, or SiC.
Various full-color image patterns are formed and used as thermal transferred images under the condition of the thermal head. Among many conditions, under the condition where a dark painted image and a half tone image are printed close to each other, there was observed a problem of staining by tailing, seemingly due to the influence of the buildup temporarily deposited in the area where the thermal head and the back face of the thermal-transfer sheet become in contact with each other, occurring on the area of half tone image, when the heat energy applied to the thermal head varies rapidly from high to low energy.
In the thermal transfer-recording method, it is possible to print images different in size if the size of the image is smaller than the width in the main scanning direction of thermal head, by using a thermal-transfer sheet and an image-receiving paper similar in width. When an image having a width of (W1) is printed on multiple image-receiving papers with a thermal-transfer sheet and then an image having a broader width of (W2) on the image-receiving paper with the thermal-transfer sheet, a problem of image lack separated by a width of (W1) occurs (see FIG. 2). As shown in FIG. 3, the problems occurs, because edge buildup 33 depositing on the thermal head 30 at the both terminals as separated by a distance of the image-receiving paper width of (W1) during printing at an image-recording width of (W1) prevents heat transfer in the terminal crust areas during printing at a broader paper width of (W2).
Generally in forming a print having no white edge extending in the main scanning direction of thermal head 30 by the thermal transfer-recording method, an image is printed in larger size than the image-receiving paper 32 by using a thermal-transfer sheet 31 having a width wider than the image-receiving paper 32. The thermal-transfer sheet 31 in the area beyond the width of the image-receiving paper 32 is exposed to the heat from the heating unit 34 of the thermal head, but the heat of the thermal head is not used for printing. As a result, the heat-resistant protective layer fused by the heat applied to the heat-resistant protective layer of the thermal-transfer sheet 31 adheres as buildup on the position of the thermal head 30 corresponding to the terminals of the image-receiving paper.    Patent Document 1: Japanese Patent Application Laid-Open No. 61-184717    Patent Document 2: Japanese Patent Application Laid-Open No. 62-220385,    Patent Document 3: Japanese Patent Application Laid-Open No. 5-229271    Patent Document 4: Japanese Patent Application Laid-Open No. 5-229272    Patent Document 5: Japanese Patent Application Laid-Open No. 8-113647    Patent Document 6: Japanese Patent Application Laid-Open No. 8-244369    Patent Document 7: Japanese Patent Application Laid-Open No. 10-297124